1. Field of the Invention
The invention relates to an electro-magnetic relay and a cover used therefor, and more particularly to an electro-magnetic relay required to have quick response to high frequency signals, and a cover used therefor.
2. Description of the Related Art
One of conventional electro-magnetic relays is illustrated in FIGS. 1 and 2. With reference to FIG. 1 which is an exploded perspective view of the conventional electro-magnetic relay, the illustrated electro-magnetic relay is comprised of a coil assembly 100, an armature block assembly 200, and a base assembly 300.
The coil assembly 100 includes a core 15 covered with coil spool 11 except opposite ends acting as magnetic poles, a coil 12 wound around the core 15, and a permanent magnet 13 inserted into a hole formed at the center of the coil spool 11, and thus located at the center of the core 15. The coil spool 11 is composed of insulating material.
The armature block assembly 200 includes an armature 20, swingable springs 22a and 23a each having a contact 22b and 23b at a distal end thereof, and an armature block 250 made of insulating material and formed integral with the armature 20 and the springs 22a and 23a. The armature 20 moves like a seesaw by applying a current to the coil 12 or stopping application of a current to the coil 12. In such seesaw movement of the armature 20, a center portion thereof located on the permanent magnet 13 acts as a fulcrum. The swingable springs 22a and 23a are connected to hinge springs 22 and 23, respectively.
The base assembly 300 includes fixed contacts 30a and 31a facing to the swingable contacts 22b, fixed contacts 32a and 33a facing to swingable contacts 23b, fixed contact terminals 30, 31, 32, and 33 on which the fixed contacts 30a, 31a, 32a, and 33a are mounted, neutral terminals 38 and 39, coil terminals 34, 35, 36, and 37, and a box-shaped insulating block (no reference numeral) formed integral with the fixed contact terminals 30, 31, 32, and 33, the neutral terminals 38 and 39, and the coil terminals 34, 35, 36, and 37. The fixed contact terminals 30, 31, 32, and 33 are formed to outwardly project to thereby act as relay terminals.
Ends of the coil 12 are electrically connected to welding portions 11a buried in the coil spool 11, and further electrically connected to coil terminals 34, 35, 36, and 37 by welding. The hinge springs 22 and 23 of the armature block assembly 200 are electrically connected to the neutral terminals 38 and 39 of the base assembly 300, respectively, by welding.
FIG. 2 is a perspective view showing how the electro-magnetic relay is assembled. The armature block assembly 200 and the coil assembly 100 are assembled to the base assembly 300. The armature block assembly 200, the coil assembly 100, and the base assembly 300, which are assembled to one another, are covered with a cover 400 made of plastics. Gaps between the cover 400 and the base assembly 300 are filled with electrically insulating sealing material such as epoxy resin. Thus, there is completed the electro-magnetic relay.
The conventional electro-magnetic relay having the above-mentioned structure has a problem of poor response to high frequency signals input to contacts. This is because the cover 400 is made of plastics, and thus, does not have shielding characteristic, which means that it is not possible to match with a transmission path with respect to characteristic impedance.
In order to overcome this problem, Japanese Unexamined Patent Publication No. 4-263508 having been published on Sep. 18, 1992 has suggested an electrically insulating cover to which shield characteristic is provided.
FIGS. 3 and 4 illustrate the cover suggested in the above-mentioned Publication. FIG. 3 is a perspective view of the cover with portions cut away, and FIG. 4 is a perspective view of the cover, as viewed from downwardly.
As illustrated in FIG. 3, a piezoelectric electronic part P is mounted on a base B, and is covered with an electrically insulating cover C.
As illustrated in FIG. 4, the cover C is formed at an entire upper inner surface with a shield electrode C1, and at a part of side inner surfaces with shield electrodes C2. Furthermore, the cover C is formed at an external surface thereof with a external electrode C3 and a shield electrode C4. The shield electrodes C4 are designed to electrically connect with the shield electrodes C1 and C2. The reason why the cover C is formed at an external surface thereof with the external electrode C3 and the shield electrodes C4 is to use an electro-magnetic relay with the cover C, as a chip component.
However, the above-mentioned cover is accompanied with a problem that since the external electrode C3 is formed on an external surface of the cover C, it is impossible to ensure a high breakdown voltage between the cover C and the external electrode C3, or a high breakdown voltage between the shield electrodes C1 to C3 and the external electrode C3.
If the electrically insulating cover illustrated in FIGS. 3 and 4 were used for an electro-magnetic relay, there would be caused a big problem on safety that it is impossible to have a high breakdown voltage between a cover and coil terminals, and between a cover and contact terminals.
In addition, the cover C illustrated in FIGS. 3 and 4 is formed at just a part of an inner side surface thereof with the shield electrode C2, though the cover C is formed at an entire inner upper surface with the shield electrode C1. As a result, the cover C is accompanied with a problem of insufficient shield characteristic against electro-magnetic waves entering through a sidewall of the cover C.